1. Field of the Invention
This invention relates to the packaging of semiconductor devices, and in particular to a flip chip ball grid array package and a method for creating a flip chip package.
2. Description of the Related Art
Flip chip ball grid array packages are becoming increasingly popular due to the ever increasing demands for higher pin counts. An example of a prior art flip chip ball grid array package is illustrated in FIG. 1. As shown, bond pads 40 on a die 42 are electrically coupled to a substrate 44 by bumps 46. The substrate 44 comprises electrically conductive vias 48, which are often created by lasers, that electrically couple the bond pads 40 to solder balls 49. In turn, solder balls serve to electrically couple the package to a printed circuit board. A stiffener 48 helps prevent warpage of the substrate 44 and also serves to support a heat spreader 50.
To create the package shown in FIG. 1, the entity performing the packaging will mount the die 42 to the substrate 44 and dispense the underfill epoxy. Thereafter, the stiffener 48 is mounted to the substrate 44 and finally, the heat spreader 50 is mounted to the stiffener 48.
There are various drawbacks to the assembly procedure described above. First since the current trend in substrate manufacturing is towards denser substrates with thinner outlines, it is difficult to manufacture a substrate with a small amount warpage. A relatively large amount of substrate 44 warpage, especially in the die 42 area, results in the die bumps 46 not mating properly with pads on the substrate 44, which in turn may result in an open joint. Also, CTE mismatch between the die 42 and the substrate 44 after the underfill epoxy has been dispensed causes both the substrate 44 and the die 42 to warp. Such warpage is a critical aspect of the assembly process since it governs the coplanarity of the package and, as previously mentioned, affects interconnections between the substrate 44 and the die 42. Second, aligning the stiffener 48 and heatspreader 50 with the substrate 44 to within the current JEDEC standard (+/-0.2 mm) is difficult because the stiffener 48 and heatspreader 50 move on top of the epoxy when it is cured. Tooling such as boat required to align the stiffener 48 and heatspreader 50 during the curing of epoxy cure is unreliable.
The foregoing attachment problems result in poor yields during the solder ball attach process because the ball fluxing and placement are referenced from the edge of the package and if the edge of the package has a wider tolerance than the ball pad, then the flux is not registered on the ball pad and leas to a non-wet solder ball. Also, the misalignment causes a poor fit for the test sockets and has resulted in test yield losses.